1. Field of the Invention
The invention is directed to a process for treating a container comprising a normally solid, plastic surface with a sulfur-containing compound at such conditions that the permeability of the container is substantially reduced, as compared to previously known processes. The invention is also directed to the container produced in the process.
2. Discussion of Related Art
Containers made of normally solid, plastic (i.e., made from polymer) materials are known in the art. Such containers have a variety of uses, e.g., they can be used for storing or transporting industrial chemicals, hydrocarbons, such as fuels, and similar liquids.
Plastic containers have several advantages over metal containers in a variety of applications, including transporting and storing hydrocarbon liquids. For example, plastic containers are more economical to manufacture and are more light-weight than metal containers, such as metal automobile gasoline tanks, thereby adding to fuel efficiency of the automobiles in which they are used. Additionally, they can be molded into a single piece, seamless article, thereby eliminating joined surfaces, such as seams, usually required in metal containers. Seams are particularly vulnerable to leaks and may become corrosion initiation points.
Additionally, plastic containers are not susceptible to corrosion by liquids. In contrast, metal containers are particularly susceptible to corrosion by liquids which they transport, particularly alcohols and flexible fuels, such as mixtures of gasoline and alcohol. For example, it has been estimated that metal, such as terne plate steel, fuel tanks for automobiles can last only about 3 to about 5 years if they are used to transport a fuel comprising a mixture of gasoline and methanol.
It was recognized in the art that plastic containers may be permeable to vapors produced by some of such liquids. For example, polyethylene, particularly high density polyethylene (HDPE), and high molecular weight, high density polyethylene (HMW HDPE), containers have been used as fuel tanks for hydrocarbon liquids, such as gasoline and alcohols and other fuels, including "flexible fuels." The term "flexible fuels" includes mixtures of hydrocarbon fuels, such as gasoline and alcohols, which are blended for specific purposes, such as reduction in emissions to meet environmental standards.
Accordingly, various treatments have been proposed to decrease the permeability of plastic containers to the vapors. For example, Walles U.S. Pat. Nos. 3,613,957 and 3,740,258, disclose enclosure members fabricated of non-aromatic, hydrocarbon polymers which are rendered substantially impermeable to gasoline, hydrocarbons and other organic materials by treating the enclosure members (containers) with a sulfonating agent, such as sulfur trioxide (SO.sub.3), to decrease permeability thereof. The containers are treated to such an extent that, in the preferred embodiments, the sulfonate groups are present on the surface or surfaces of the containers in the concentration of about 0.06 to about 20, preferably from about 0.1 to about 1 milligram (mg.) of sulfur trioxide equivalents per square centimeter (e.g., see the '258 Patent). According to the teachings of both of these patents, the containers are treated with the sulfonating agent after they are fabricated by any suitable means, such as in a blow-molding process.
Walles et al., U.S. Pat. No. 4,861,250, disclose an in-mold sulfonation system for sulfonating the surface of plastic articles, such as plastic containers, during molding. In the sulfonation process, sulfur trioxide at a pressure exceeding that of the mold, is introduced into the mold, such as a blow-mold, immediately before or immediately following full expansion of the formed plastic article in the mold cavity. The sulfonation step is followed by the step of neutralization with a gaseous neutralizing agent, such as ammonia gas. The sulfonation step is conducted at a temperature of 160.degree. to 80.degree. C., because at that temperature a minimum amount of leachable salt is produced. Adequate sulfonation levels are described to be equal to or greater than 200 micrograms of sulfur trioxide (SO.sub.3) per square centimeter. Walles et al. also state that 90% reduction in gasoline permeation rate of the blow-molded container is sufficient. That level is reached at about 22 micrograms of sulfur (S) per square centimeter (cm.sup.2) at 72.degree. F. and 50% relative humidity or at about 46 micrograms S per cm.sup.2 at 100.degree. F.
Walles, U.S. Pat. No. 4,615,914, discloses a method of treating plastic containers to provide increased barrier properties to organics and gases, such as oxygen. The method comprises treating the interior surface of the container with sulfur trioxide gas produced by inserting into the interior of the container a solid, particular material, which, upon heating, forms sulfur trioxide gas. The container is heated by a microwave energy or by any other suitable means to form sulfur trioxide gas from the solid material. After the sulfonation treatment, the sulfonated surface is treated with a suitable material to neutralize the sulforic acid groups formed on the polymer to prevent the reaction of the acid groups with materials which are later placed into the container. An example of a suitable neutralizing material is ammonia gas (NH.sub.3).
Walles et al., U.S. Pat. No. 2,786,780, and Walles, U.S. Pat. Nos. 2,832,696, 2,937,066 and 4,775,587, also disclose methods of sulfonating plastic, such as polyethylene, materials for the purposes of decreasing permeability thereof to various materials, or to provide a suitable outside surface for adhering dyes or other coatings thereto.
Staudinger et al., U.S. Pat. No. 2,400,720, disclose the treatment of plastic surfaces or objects with concentrated sulfuric acid, fuming sulfuric acid, sulfur trioxide or chlorsulphonic acid to render such surfaces hydrophilic, thereby enabling the application of water-soluble dyes thereto.
Lundbert et al., U.S. Pat. No. 4,157,432, disclose a bulk sulfonation process comprising mixing a hydrocarbon polymer having olefinic unsaturation or aromatic moieties, or both, by mechanical means, in the absence of a solvent for the polymers, with a sulfonation reagent and at a sufficient temperature and time to effect the desired degree of sulfonation.
Bock et al., U.S. Pat. Nos. 4,014,831 and 4,220,573, disclose ionic polymer compositions which include a metal-neutralized sulfonated polymer plasticized with a preferential plasticizer.
Thus, plastic containers have heretofore been sulfonated to decrease permeability thereof after they were fabricated, e.g., in a blow-molding process. They were sulfonated by contacting the containers for a suitable period of time, e.g., about 150-160 seconds, at ambient temperature with a suitable mixture of a sulfonating agent (also referred to herein as a "sulfonating agent" or a "sulfur-containing compound") and an inert gas comprising, e.g., about 15% by mole of the sulfonating agent. Subsequently, the sulfonated surface was neutralized by a suitable neutralizing agent and washed with an aqueous liquid, such as water, to remove any leachable salts. The resulting container had surface sulfonation levels of about 200 to about 400 micrograms of sulfur per square inch (mcg/in.sup.2). The thus-sulfonated containers had average permeability to unleaded gasoline of about 0.04 g/hr or to flex fuels (a blend of unleaded gasoline and methanol-M-10 or M-15) of about 0.08 g/hr.
The heretofore-available sulfonated plastic containers may be unable to meet newly-proposed environmental emission regulations, particularly when such containers are used for transporting flexible fuels, such as mixtures of gasoline and alcohol(s). For example, up to the present time, automobile industry has required plastic fuel tanks having sulfonation levels of between about 200 to about 400 micrograms of sulfur per square inch (mcg/in.sup.2). Such levels are considered adequate for meeting current U.S. environmental emission standards limiting hydrocarbons emissions from the entire car to a maximum of 2 gram/2hr (g/2hr), if the car is powered by unleaded gasoline, as measured by the Shed Test, described in detail in SAE J171, June 1982, incorporated herein by reference in its entirety. Such sulfonation levels are also considered adequate for meeting current European and Japanese environmental regulations limiting the amount of hydrocarbon (e.g., unleaded gasoline) emissions from an automobile fuel tank to not greater than 20 grams/24 hrs. However, newly-proposed environmental emission standards are considerably more stringent. Additionally, due to recently enacted environmental legislation, such as Clean Air Act, limiting permissible level of hydrocarbon emissions, there is increased interest in the use of flexible fuels which are believed to produce lower emission levels in internal combustion engines. Plastic fuel tanks are more suitable than metal tanks for use as containers for such flexible fuels for the reasons discussed above, e.g., because plastic fuel tanks are not susceptible to corrosion. However, plastic fuel tanks are more permeable to flexible fuels vapors than to pure hydrocarbons, such as gasoline. Accordingly, plastic fuel tanks having sulfonation levels of about 200 to about 400 mcg/in.sup.2 of sulfur, produced in accordance with the previously-used sulfonation processes, may be unable to meet the newly-proposed environmental restrictions.
Therefore, it is important to provide a plastic container, such as a plastic fuel tank, which can meet the newly-proposed environmental emission standards even with new flexible fuels, such as mixtures of hydrocarbon fuels and alcohols, e.g., gasoline and methanol or ethanol.